This invention pertains to replication of diffractive optical elements embodied as surface relief structures and profiles and particularly to replication of higher quality and more durable optical elements than those in the related art.
Others have used a dry photopolymer embossing (DPE) method to produce high fidelity and high image-quality replicas of diffractive optical elements on "plastic" and "glass" substrates. A film is formed on the substrate and is embossed with a stamper to provide an embossed film that is a replica of the original diffractive element. Since the embossed film is made from a polymeric or like embossable material, the embossed film does not have good durability under affecting environmental conditions such as temperature, humidity, ultraviolet light, aerosols, physical handling, wear, and so forth. The embossed film on the substrate may lose its adhesion to the substrate or the lose fidelity of its profile.
Diffractive optical elements, sometimes referred to as surface relief holograms, kinoforms, binary optics, or phase gratings, are used to replace or enhance conventional optical components in a variety of applications, such as head-mounted displays, projection displays, photocopiers, optoelectronic modules for data communications, optical storage devices, electronic imaging sensors and systems, laser systems, and ophthalmic products for the vision impaired. Generally, the optical functions implemented in diffractive elements can be classified as either imaging or nonimaging. Examples of imaging functions are focusing power, aspheric aberration correction, chromatic aberration correction, distortion correction, and athermalization. Examples of nonimaging functions are spectral filters, condensing microlenses, spot array generators (Damann gratings), diffusing screens, and zero-order grating structures used for antireflective devices, phase retardation devices, or polarizing devices. For many applications involving diffractive optics as either imaging or nonimaging elements, the optical performance must not degrade in the presence of commonly encountered environmental conditions such as humidity and temperature variations and exposure to ultraviolet radiation or aerosols. Further, when used to implement imaging functions, the diffractive elements must maintain high image quality in the optical wavefronts transformed by the diffractive surface.
Replication of diffractive optics is widely recognized as necessary to achieve affordable mass producibility. A film of photopolymer is formed on the substrate and is embossed with a stamper to provide an embossed film that is a replica of the original diffractive element. A related art dry photopolymer embossing procedure for replication of diffractive optic elements is related by E. I. Du Pont de Nemours and Company (Du Pont) of Wilmington, Delaware. Such procedure and certain substances used in such related art replication is disclosed in a U.S. Pat. No. 5,279,689, by Felix P. Shvartsman, issued Jan. 18, 1994, and entitled "Method for Replicating Holographic Optical Elements," (hereafter Shvartsman) which is hereby incorporated by reference in the present description. Shvartsman discloses both a dry photopolymer embossing procedure and a photopolymeric material (trade name SURPHEX) capable of replicating with high fidelity diffractive elements having high aspect ratio. SURPHEX is capable of replication of very high aspect ratio features (in the range of 20:1) with very high fidelity (with shrinkage less than 0.1 percent) while maintaining a high optical finish and wavefront quality (of about .lambda./10). Shvartsman further discloses that the substrate may be either polycarbonate, polymethylmethacrylate (PMMA), or glass. However, subsequent research has demonstrated that the photopolymeric composition disclosed adheres only to the plastic substrate materials (polycarbonate and PMMA) and does not adhere to glass.
J. A. Cox and F. P. Shvartsman, "Image Quality Assessment of Diffractive Optical Elements Replicated in Surphex," in Diffractive Optics, Vol. 11, 1994, OSA Technical Digest Series (Optical Society of America, Washington, DC, 1994), pp. 346-9 (hereafter Cox and Shvartsman), provide experimental data to demonstrate that ideal, diffraction-limited imagery is feasible on PMMA substrates using the dry photopolymer embossing procedure and SURPHEX photopolymer disclosed by Shvartsman. Cox and Shvartsman further demonstrate that observed aberrations degrading image quality in the replicas are caused by surface irregularity in the plastic substrate itself and are not caused by any imperfection in the photopolymer film or the embossing procedure.
It should be noted that in order to achieve both high fidelity and high image quality in the replicated diffractive element, the photopolymer film must possess special characteristics with respect to shrinkage and uniformity of refractive index. First, there is some shrinkage in the embossed film after curing. Shrinkage changes the dimensions of the surface profile embodying the diffractive element, often in an unpredictable manner, and causes degradation in the diffraction efficiency of the replica, leading to the undesirable diversion of light into higher diffractive orders. Shrinkage can also cause warpage in the substrate carrying the embossed photopolymer film, and the warpage introduces undesired optical aberrations, such as astigmatism and coma, in the image quality. Experience has shown that acceptable optical performance is achieved when shrinkage is less than one percent. F. P. Shvartsman, "Replication of Diffractive Optics," in Critical Reviews on Diffractive and Miniaturized Optics, Vol. CR49 (SPIE Press, Bellingham 1994), pp. 165-86 (hereafter Shvartsman's article), has shown that shrinkage in SURPHEX, the photopolymer disclosed in Shvartsman, is less than one-tenth of a percent. Secondly, in order to achieve high image quality, the cured photopolymer film must exhibit good spatial uniformity in its index of refraction. Variations in the refractive index can cause serious degradation in image quality, as noted by Cox and Shvartsman. Although it has not been possible to establish precise bounds on acceptable variation in the refractive index, the data of Cox and Shvartsman demonstrate that the photopolymer disclosed by Shvartsman does meet the requirement while other materials, such as ultraviolet-curable optical epoxies, do not.
In experiments conducted by J. A. Cox, unpublished test results recorded in Honeywell Data Book, "Diffractive Optics No. 8", pp. 22-50, 18 Oct. 1994--11 Nov. 1994 (hereafter Cox) relating to humidity and temperature susceptibility tests performed on diffractive optical elements replicated in SURPHEX on PMMA substrates (2 each) and fused quartz substrates (2 each) in accordance with MIL-STD-810B, Method 507, Procedure I, using both the dry photopolymer embossing procedure and the photopolymer disclosed by Shvartsman, Cox duplicated the results reported in Shvartsman's article, for plastic (i.e., PMMA) substrates. Cox also demonstrated that the same photopolymer does not adhere to glass (fused quartz) substrates. Finally, Cox subjected replicas of a diffractive optical element described by Cox and Shvartsman to a standard humidity and temperature environmental test prescribed for optics (MIL-STD 810B, Method 507, Procedure I), and Cox observed degradation in both the physical and optical properties of the replicas. The most serious degradation in physical properties observed were etching in the surface of the photopolymer film and changes in the dimensions of the surface profiles features. The most serious degradation in optical properties observed were increased scatter and a decrease in diffraction efficiency.
Thus, the prior art disclosed by Shvartsman provides a means of replicating diffractive elements with high fidelity and high image quality on plastic substrates, wherein the image quality is limited by the surface quality of the plastic substrate and the photopolymer film is not durable under common environmental conditions.